Your support team can bring so much to your experience by being a calm, strong, and comforting presence. By simply being attentive, quiet, and ever-present, they will give you a feeling of peace and strength. Having them there will change the environment in which you birth, and anyone who comes into your space will feel that change and be more inclined to act accordingly.
Conversely, having the wrong people on your team can have adverse affects on you and your environment. Someone who is loud, inattentive, argumentative, uncomfortable, or simply someone who you are not close to, can cause you to feel uncomfortable or distracted and take the focus off the task of laboring.
So choose wisely. Choose people you know who will contribute to your success. Avoid having extra people there as guests or simply to watch the birth.